Ruby 1.9 Fibers has got me reading about Coroutines.
Thought I should put all my understanding somewhere, as I read and understand coroutines in more depth.

Most of the content in this post just a aggregation of various sources.

Coroutines are program components that allow multiple entry points and can return any number of times. Coroutines belong to a category of programming construct called Continuations.

All programming languages have one way or another to handle control flow. Within a control flow there is an associated state. This state is information like value of a variable etc. Callstack is one of the most popular way to store this information. Every method has its own call stack and this stack is erased once the method returns either normally or through exceptional Flow.

In a Coroutine this is not the case. We can suspend and resume execution without loosing the stack.

Types of Coroutines:

1. Symmetric Coroutines: A symmetric coroutine can use one function to yield and another to resume. Example: Lua

2. Asymmetric Coroutines: They are also called as semi-coroutines. The choice for the transfer of control is limited. Asymmetric Coroutines can only transfer control back to their caller. Example: Ruby 1.9 Fibers

The main reason why coroutines are making the limelight again is because of concurrency. In my humble opinion, concurrency is reviving many of the well known but forgotten programming concepts back.

To take the example of ruby, most of us are aware of the Global Interpreter Lock. Threading in ruby is totally useless because ultimately all thread run as part of the same OS thread, which means there no true concurrency. Fibers in ruby are very similar to threads but are light weight threads. They can scheduled, suspended and resumed as per the programmers choice.

Coroutines can be used to construct the actor model of concurrency. This is the same model used by Erlang. Revactor is a very nice implementation of the actor model in ruby.